Taken directly from FIFA Laws of the Game:
"Sending-Off Offences: A player is sent off and shown the red card if he commits any of the following seven offences:
5. denies an obvious goal-scoring opportunity to an opponent moving towards the player’s goal by an offence punishable by a free kick or a penalty kick."
If your description of the foul is correct, that should be a red card, without question.
